Tanks with such a floating cover are presently used on a wide scale, in particular for storing liquid materials of the petrochemical type, e.g. mineral oil and mineral oil products. In such tanks heretofore, the edge gap between the outer perimeter of the cover and the inner wall of the tank has been sealed to prevent upward escape of the liquid, with the aid of a device comprised of a lever bar with counter-loads, sliding plates, and flexible wiper-type strips.
However, the known sealing means will not suffice to meet more stringent future environmental protection requirements, and thus it will be necessary to retrofit current tank apparatuses or to construct completely new tanks. According to the state of the art prior to the present invention, essentially two alternatives are presented for consideration: Either
the method of gas displacement, or
(for liquid materials of suitable hazard classes) the use of a fixed-roof tank with a floating cover, wherewith air exhaust and gas emissions are sent to an apparatus which performs waste gas disposal and/or vapor recovery.
Both of these alternatives are attended by significant drawbacks. The method of gas displacement does indeed avoid emissions of pollutants, but it is not practicable in a number of applications, e.g. intermediate storage of crude oil in crude-oil tanks, where this oil is in the process of being pumped over relatively long pipelines. In the case of fixed-roof tanks with associated disposal and recovery apparatuses for the gases and vapors which are produced, in addition to high construction costs one is faced with major operating difficulties, because said disposal- and recovery apparatuses will periodically require maintenance, and in such cases (as well as in cases of unexpected repairs) the tanks will have to be emptied. Furthermore, the continuous operation of such apparatuses consumes substantial energy, which itself is detrimental to the environment.
Accordingly, the underlying problem of the present invention is to devise means of controlling emissions of pollutants to levels heretofore only attainable with the gas displacement method, but
without changing the customary floating cover tank structure which is of simple design and is easy and inexpensive to operate, and
without adding costly additional equipment.